Electromagnetic wave detector

ABSTRACT

An electromagnetic wave detector includes a conversion element for converting incident electromagnetic waves or high energy radiations into an electric charge, a storage capacitor for storing the electric charge produced by the conversion element, a thin film read transistor connected to the storage capacitor, and a thin film reset transistor also connected to the storage capacitor. To the gates of the read and reset thin film transistors are applied ON and OFF voltages at predetermined timings and these voltages are set to values such that any excessive electric charge produced in the storage period is discharged by way of the thin film reset transistor, not by way of the thin film read transistor, in the same storage period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electromagnetic wave detector for detectingelectromagnetic waves or high energy radiations such as ultravioletrays, infrared rays, rays of visible light, X rays, α rays or γ rays.

2. Related Background Art

Electromagnetic wave detectors adapted to directly or indirectly convertelectromagnetic waves or high energy radiations such as ultravioletrays, infrared rays, rays of visible light, X rays, α rays or γ raysinto an electric charge by means of a semiconductor article and read thestored electric charge find applications particularly in the field ofimaging apparatus.

U.S. Pat. No. 5,811,790, Japanese Patent Application Laid-Open No.09-288184, U.S. Pat. No. 5,856,699, Japanese Patent ApplicationLaid-Open No. 09-260626 and other patent documents describe anelectromagnetic wave detector adapted to convert visible light obtainedby transforming X rays or some other high energy radiations into anelectric charge and read the stored electric charge.

U.S. Pat. No. 5,391,881 describes an electromagnetic wave detectoradapted to directly convert electromagnetic waves or high energyradiations such as ultraviolet rays, infrared rays, rays of visiblelight, X rays, α rays or γ rays into an electric charge.

FIGS. 14A and 14B of the accompanying drawings show a schematic crosssectional view and a schematic plan view of a known detector having alayered structure of monocrystalline bulk X ray detecting sections andmonocrystalline readout ICs.

Referring to FIGS. 14A and 14B, as high energy electromagnetic waves 108such as X rays enter the detector, an electric charge is generated insemiconductor substrates 106 typically made of Si, GaAs, CdTe or HgI₂and transferred to readout circuits 116 of integrated circuit chips 110a and 110 b by way of electrodes 114, bumps 120 and electrodes 119.Electrodes 134 a through 134 e and electrodes 130 a through 130 d aswell as bumps 136 are provided to connect the semiconductor substrates106 and the integrated circuit chips 110 a and 110 b.

U.S. Pat. No. 5,198,673 describes a direct type sensor equipped withprotection diodes. FIG. 15 of the accompanying drawings is a schematicblock diagram of the read/reset circuit of a direct type sensor 160having protection diodes as disclosed in the above patent document.Referring to FIG. 15, there are shown scan switches 222 a and 222 bconnected to scan wires 220 a and 220 b and output wires 230, the latterby turn being connected to sample-and-hold amplifiers (read circuits)235 and reset circuits 237. The scan switches are also connected tosensors 210, high voltage sources 212, storage capacitors 214 andovervoltage protection elements (protection diodes) 240.

With an electromagnetic wave detector of the above described type havinga circuit configuration that comprises storage capacitors for storingthe generated electric charge, from which the stored electric charge isread out, a residual electric charge can be left in the detector afterreading the electric charge from the storage capacitors. Then, theresidual electric charge is added to the electric charge stored in thenext cycle to give rise to a problem of after image when moving imagesare involved. Additionally, if the storage capacitors are loadedexcessively with electric charge, the excessive charge can leak out tothe read circuit to give rise to a phenomenon like that of blooming in aCCD image sensor. This phenomenon is particularly remarkable when thedetector detects electromagnetic waves such as X rays whose energy levelis higher than visible light.

Additionally, when transistors for reading the stored electric chargeare formed in a bulk by using a monocrystalline wafer, electric chargesunintentionally generated by high energy electromagnetic waves in thebulk can adversely affect the transistors to make them no longer operateproperly.

SUMMARY OF THE INVENTION

In view of the above circumstances, it is therefore an object of thepresent invention to provide an electromagnetic wave detector thatoperates more excellently than comparable known detectors and adapted todetect high energy electromagnetic waves effectively and efficiently.

Another object of the present invention is to provide an electromagneticwave detector having a large imaging area at low cost than ever.

Still another object of the present invention is to provide anelectromagnetic wave detector that can effectively prevent any afterimage and leakage of electric charges from taking place and operatewithout errors even when high energy electromagnetic waves enter it.

According to the invention, the above objects are achieved by providingan electromagnetic wave detector comprising conversion elements forconverting incident electromagnetic waves or radiations into an electriccharge; storage capacitors for storing the electric charge produced bythe conversion elements; thin film read transistors connectedrespectively to the corresponding storage capacitors and each having agate to which ON and OFF voltages are applied respectively in readoutand storage periods; and thin film reset transistors connectedrespectively to the corresponding storage capacitors and each having agate to which ON and OFF voltages are applied respectively in reset andstorage periods, the OFF voltage applied to the gates of the thin filmreset transistors being set to a value closer to the ON voltage appliedto the gates of the thin film reset transistors than the OFF voltageapplied to the gates of the thin film read transistor.

In another aspect of the invention, there is also provided anelectromagnetic wave detector comprising conversion elements forconverting incident electromagnetic waves or radiations into an electriccharge; storage capacitors for storing the electric charge produced bythe conversion elements; and thin film reset transistors connectedrespectively to the corresponding storage capacitors and each having agate to which ON and OFF voltages are applied respectively in reset andstorage periods, any excessive electric charge being discharged by wayof the thin film reset transistors in each storage period.

Preferably, the conversion elements are adapted to absorbelectromagnetic waves showing an energy level higher than visible lightand convert them into an electric charge.

Preferably, the thin film read transistors and the thin film resettransistors have a non-monocrystalline semiconductor layer formed on aninsulating substrate.

Preferably, the thin film read transistors and the thin film resettransistors are formed on an insulating substrate, and the conversionelements are formed on a substrate different from the insulatingsubstrate and electrically connected to the thin film read transistorsand the thin film reset transistors.

Preferably, the conversion elements comprise a semiconductor substratehaving two opposite surfaces for converting electromagnetic waves intoan electric charge, a common electrode arranged on the one surface ofthe semiconductor substrate and a plurality of electrodes formed on theother surface of the semiconductor substrate and separated from eachother in correspondence to a plurality of two-dimensional pixels; thethin film read transistors and the thin film reset transistors areformed on an insulating substrate such that unit cells each includingone of the thin film read transistors and one of the thin film resettransistors are arranged on the insulating substrate in correspondenceto the pixels; and the semiconductor substrate and the insulatingsubstrate form a layered structure and the plurality of electrodes andthe unit cells are electrically connected between the substrates.

Preferably, the semiconductor substrate is provided in plurality andthey are arranged two-dimensionally on the insulating substrate to forma layered structure and the common electrodes of the semiconductorsubstrates are mutually short-circuited.

Preferably, a high voltage is applied to the common electrode of theconversion elements and a shielding conductor is arranged near thecommon electrode.

Preferably, the thin film read transistors and the thin film resettransistors are formed on an insulating substrate provided with a drivercircuit for driving the thin film read transistors and the thin filmreset transistors and with a read circuit for reading signals from thethin film read transistors.

Preferably, the voltage applied to the source/drain of each of the thinfilm read transistors is so selected that the potential differencebetween the source and the drain of the thin film transistor is at least1V or more in the initial stages of a reading operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of an embodiment ofelectromagnetic wave detector according to the invention, showing itscircuit configuration.

FIG. 2 is a schematic circuit diagram of a pixel of the embodiment ofelectromagnetic wave detector of FIG. 1, showing its circuitconfiguration.

FIGS. 3A, 3B and 3C are schematic cross sectional views of thin filmtransistors that can be used for the purpose of the invention.

FIG. 4A is a circuit diagram of a thin film transistor that can be usedfor the purpose of the invention and FIG. 4B is a graph showing thecharacteristic performance of the thin film transistor of FIG. 4A.

FIG. 5 is a basic drive timing chart of the embodiment ofelectromagnetic wave detector of FIG. 1.

FIG. 6 is a drive timing chart of the embodiment of electromagnetic wavedetector of FIG. 1.

FIG. 7 is a graph showing the characteristic of ON resistance that canbe used for the purpose of the invention.

FIG. 8 is a graph showing the V_(d) dependency of the ON resistance of athin film transistor that can be used for the purpose of the invention.

FIG. 9 is a schematic circuit diagram of a pixel of another embodimentof electromagnetic wave detector according to the invention, showing itscircuit configuration.

FIG. 10 is a schematic cross sectional view of a typical example ofelectromagnetic wave detector according to the invention.

FIG. 11 is a schematic cross sectional view of another typical exampleof electromagnetic wave detector according to the invention.

FIGS. 12A and 12B are a schematic plan view and a schematic crosssectional view of still another typical example of electromagnetic wavedetector according to the invention.

FIG. 13 is a schematic block diagram of a medical diagnostic systemcomprising an imaging device realized by using an electromagnetic wavedetector according to the invention.

FIGS. 14A and 14B of the accompanying drawings show a schematic crosssectional view and a schematic plan view of a known detector having alayered structure of monocrystalline bulk X ray detecting sections andmonocrystalline read ICs.

FIG. 15 is a schematic block diagram of the read/reset circuit of aknown direct type sensor having protection diodes, showing its circuitconfiguration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, an electromagnetic wave detector according to the invention will bedescribed in greater detail by referring to FIGS. 1 to 2, 3A to 3C, 4A,4B, 5 and 6 of the accompanying drawings that schematically illustratepreferred embodiments of the invention.

FIG. 1 is a schematic circuit diagram of an embodiment ofelectromagnetic wave detector according to the invention, showing itscircuit configuration.

FIG. 2 is a schematic circuit diagram of part of the embodiment ofelectromagnetic wave detector of FIG. 1, including the output circuitfor reading the signal of a unit cell.

Referring to FIGS. 1 and 2, reference numerals 1 and 2 respectivelydenote a conversion element and a storage capacitor and referencenumerals 3 and 4 respectively denote a reset transistor and a readtransistor, whereas reference numerals 5 and 6 respectively denote anoutput line and a horizontal drive control line and reference numerals 7and 8 respectively denote a reset control line and a reset transistor ofthe output line.

Reference numeral 11 denotes a reset transistor that is optionallyprovided if necessary and reference numerals 12 and 13 respectivelydenote a horizontal transfer transistor and a scan circuit, whereasreference numeral 14 denotes a reset circuit and reference numerals 15and 16 respectively denote an amplifier and an output circuit.

A unit cell that operates as a pixel comprises a conversion element 1for converting the electromagnetic wave that enters it into an electriccharge, a storage capacitor 2 for storing the signal charge from theconversion element 1, a read transistor 4 for reading the signal fromthe storage capacitor 2 and a reset transistor 3 for resetting thesignal charge.

Unit cells are arranged two-dimensionally in the form of matrix tooperate as a so-called area image sensor.

The transistors 4 of each row are selected by way of the scan circuit 13that operates on a row by row basis to read the signals from the storagecapacitors 2 of the row to the output lines 5 and then the signals areinputted to the output circuit 16 by way of the amplifiers 15 connectedto the output lines 5 so that all the read out signals are sequentiallyoutputted to output terminal OUT on a column by column basis.

Each output line 5 is reset to reference potential V_(R2) by thecorresponding output line reset transistor 8. The output circuit 16typically comprises storage capacitors showing a capacitance of C_(SH)and arranged for the respective output lines 5 and transistors 12 forconnecting the respective storage capacitors showing a capacitance ofC_(SH) to a common output line. Transfer pulses Φ_(H1), Φ_(H2), . . .are sequentially inputted to the output circuit 16 from an output scancircuit (not shown) to sequentially turn on the transistors 12 so thatsignals are read out from the storage capacitors showing a capacitanceof C_(SH) on a column by column basis to the output terminal OUT of thecommon output line and delivered to the outside.

The drive method of the above arrangement will be discussed in greaterdetail hereinafter.

Capacitance C₂ is generated by each output line 5. The capacitance C₂includes the capacitance of the crossing areas of the correspondingoutput line 5 with the horizontal control lines 6 and 7 and that of thesource (or drain) regions of the related transistors 4. In the case of alarge device having a large substrate panel and hence adapted to cover alarge area for imaging, the capacitance C₂ is large and significantlyaffects the ratio of signal S to noise N.

When the detector is formed by using a monocrystalline substrate(typically made of Si), the capacitance between the substrate and thewires is further added thereto. However, this capacitance canadvantageously be reduced to the level of tens of several pF by using aninsulating substrate such as a glass substrate even if the substrate isa panel as large as 20 cm×20 cm.

When electromagnetic waves are made to enter the conversion element 1,if the capacitance of the conversion element 1 is C₀ and the generatedelectric charge is Q, the voltage V_(S) generated in the storagecapacitor 2 (capacitance C₁) is expressed asV _(S) =Q/(C ₁ +C ₀).

Therefore, if C₁>>C₀(C₁≧10·C₀), practically V_(S)≅Q/C₁ holds true.

When reading the signal charge from the storage capacitor 2 (capacitanceC₁) to the capacitance C₂, the potential of the capacitance C₂ isV_(C2)=Q/(C₁+C₂).

Since normally C₂>>C₁, practically V_(C2)=Q/C₂ holds true.

Thus, differently stated, V_(C2)/V_(S)=C₁/C₂ holds true so that thesignal charge is read out as voltage that varies as a function of theratio of the capacitance C₁ of the capacitor 2 to the capacitance C₂.

Therefore, if the capacitance C₂ is too large, the noise of theamplifier of the reading system becomes dominant to worsen the SN ratioof the sensor.

As pointed out above, since the capacitance C₂ can be reduced by usingan insulating substrate, the use of such a substrate is advantageousparticularly when the detector has large dimensions.

FIGS. 3A, 3B and 3C are schematic cross sectional views of typical thinfilm transistors that can be formed on an insulating substrate for thepurpose of the invention.

FIG. 3A shows a so-called lower gate stagger type thin film transistor.Referring to FIG. 3A, reference numeral 21 denotes a gate electrode madeof metal, which may be aluminum, chromium or tantalum, and referencenumeral 22 denotes a gate insulating film typically made of siliconnitride, silicon oxide, aluminum oxide or tantalum oxide, whereasreference numerals 23 and 24 respectively denote a semiconductor layerof amorphous silicon or polycrystalline silicon which is adapted to forma channel and an ohmic contact layer of a highly doped N-typesemiconductor material such as amorphous silicon or microcrystallinesilicon and reference numeral 25 denotes a source/drain electrode madeof metal such as aluminum or titanium.

FIG. 3B shows another so-called lower gate stagger type thin filmtransistor. The arrangement of FIG. 3B differs from that of FIG. 3A inthat it additionally comprises a channel protection layer 26 forprotecting the semiconductor layer 23 arranged above the gate electrode21. The channel protection layer 26 is made of an insulating materialsuch as silicon nitride or silicon oxide that shows an etching ratelower than that of the ohmic contact layer 24.

The arrangements of FIGS. 3A and 3B are advantageous when thesemiconductor layer is made of an amorphous material such ashydrogenated amorphous silicon.

FIG. 3C shows a so-called upper gate co-planar type thin film transistorthat can advantageously be adopted when a polycrystalline semiconductorsuch as polycrystalline silicon or a monocrystalline semiconductor suchas monocrystalline silicon is used as semiconductor material for forminga channel 23, a high concentration impurity region 27 that makes aP-type or N-type source/drain region and, if necessary, a lowconcentration impurity region 28 for the transistor. In FIG. 3C,reference numeral 21 denotes a gate electrode typically made ofpolycrystalline silicon or metal and reference numeral 22 denotes a gateinsulating film typically made of silicon oxide, whereas referencenumeral 25 denotes a source/drain electrode and reference number 29denotes an insulating film typically of silicon oxide or siliconnitride.

The mobility of carriers of a thin film transistor having a channelregion formed by using a non-monocrystalline semiconductor such asamorphous semiconductor or polycrystalline semiconductor is low ifcompared with that of a thin film transistor whose channel region isformed by using a monocrystalline semiconductor. However, defectivegrain boundaries and dangling bonds that causes the above drawbackoperate to trap any electric charge that is unintentionally generated byhigh energy rays entering the transistor so that a thin film transistormade of a non-monocrystalline semiconductor provides an advantage thatit scarcely commits operation errors.

In this embodiment, at least the reset switch 3 and the read switch 4are prepared by using a thin film transistor of any of the abovedescribed types. At least the reset switch 3 and the read switch 4 arepreferably prepared by way of a same film forming process. If necessary,thin film transistors may also be used for the transistor 8, the scancircuit 13, the reset circuit 14 and the output circuit 16.

Now, the operation of the transistor 3 that is a thin film transistor ofthis embodiment will be described.

After reading the electric charge from the capacitance C₂, an electriccharge of Q·C₁/C₂ remains in the storage capacitance C₁. If the storagecapacitance C₁ is 1 pF and the capacitance C₂ is 40 pF, an electriccharge of 2.5% is left and added to the capacitance of the next storagecycle to operate as noise at the next read session. In the case of amoving image, the remaining electric charge is remarkably visualized asafter image.

In this embodiment, the remaining electric charge is reset to nil tosuppress any appearance of after image by means of the transistor 3.

Additionally, in this embodiment, it is possible to discharge anyexcessive electric charge above the predetermined level of electriccharge to be stored in the storage capacitor 2 also by means of thetransistor 3. Thus, the range of electric potential (range of storedelectric charge) of the storage capacitor 2 can be defined by means ofthe transistor 3.

The initial potential of the storage capacitor 2 that is observedimmediately after a reset operation is made equal to reset referencepotential V_(R1) by applying a sufficient ON voltage to the transistor 3to turning on the latter.

After the reset, the electric charge Q flowing in from the conversionelement 1 is stored in the capacitor 2 and, if the charge of thecapacitor 2 exceeds the predetermined level, the excessive charge canleak out to the output line 5 by way of the read transistor 4. In thisembodiment, the ultimate potential (saturation potential) of thecapacitor 2 that can be produced by the electric charge is defined bythe gate voltage applied to the gate of the transistor 3.

If the voltage applied to the gate in order to define the gate potentialV_(G) is OFF voltage V_(B), the ultimate potential of the capacitor 2 isdefined by V_(B)−V_(th) (V_(th) being the threshold value of thetransistor 3). If, for instance, V_(B)=V_(th), the ultimate potential ofthe capacitor 2 is equal to zero and the range of voltage of thecapacitor 2 is defined as between V_(R1) and 0V.

Thus, in this embodiment, transistor 3 operates as reset switch and alsoas element for defining the dynamic range of the pixel.

FIG. 4A is a circuit diagram of an n-channel type thin film transistorthat can be used for the purpose of the invention and FIG. 4B is a graphshowing the characteristic performance of the thin film transistor ofFIG. 4A in terms of V_(G)−V_(S) relative to I_(D). The value of V_(th)in FIG. 4B is about 2V.

If the OFF voltage V_(B) of the gate is −1V, V_(B)−V_(th)=−3V.

The source of the transistor 3 is connected to the storage capacitor 2and reference voltage V_(R1) is applied to the drain.

If the conversion element 1 is irradiated with electromagnetic wavesunder this condition and the generated carrier electrons (−Q) are storedin the storage capacitor 2 (capacitance C₁), a negative potentialappears when V_(S)=−Q/C₁. The gate-source voltage of the transistor 3 isV_(G)−V_(S)=V_(B)+(Q/C₁) and hence it varies as a function of the storedcarriers.

More specifically, when {VB_(G)+(Q/C₁)}≧V_(th), an electric currentflows to the drain and the source voltage V_(S) stops rising anyfurther.

Under the above condition, −Q/C₁=−(V_(th)−V_(B))=−(2+1)V=−3V and hencedoes not fall under −3V.

If the electric potential of the storage capacitor 2 is not controlledin this way, it can fall remarkably to above −20V. Then, the gate/sourcevoltage of the read transistor 4 rises to give rise to a leak currentbetween the gate and the source of the transistor 4 and, if the voltagerises further, the insulating film can be damaged and become broken.

Additionally, when the source/drain voltage of the transistor 4 exceedsa predetermined level (about 20V), the electric current between thesource and the drain of the transistor 4 flows out even if the OFFvoltage is applied to completely turn off the transistor 4 so thatcarriers can flow into the vertical lines of the sensor to give rise toa so-called blooming phenomenon of CCD image sensor. Differently stated,the electric charge of the storage capacitor 2 overflows by way of thetransistor 4 so that the areas that are irradiated with electromagneticwaves particularly strongly produces a vertical influence. The abovedescribed embodiment of the invention can effectively suppress thisphenomenon.

Now, the timings of operation of the embodiment will be described byreferring to FIG. 5. The embodiment basically operates for reset,storage and readout.

Referring to FIG. 5, Φ_(VR1) denotes the voltage applied to the gate ofthe reset transistor 3 from the reset circuit 14. For instance, it maybe so arranged for Φ_(VR1) that the high level ON voltage is about +15Vand the low level OFF voltage is about +5V.

Φ_(V) denotes the voltage applied to the gate of the read transistor 4from the scan circuit 13. For instance, it may be so arranged for Φ_(V)that the high level ON voltage is about +15V and the low level OFFvoltage is about −5V.

As will be understood by comparing the OFF voltage V_(B) of the resettransistor 3 and the OFF voltage of the read transistor 4, the OFFvoltage V_(B) of the reset transistor 3 is closer to the ON voltage side(or the positive voltage side in this instance) than the OFF voltage ofthe read transistor 4.

Φ_(VR2) denotes the voltage applied to the gate of the transistor 8 forresetting the capacitance C₂ of the output line 5 to reference potentialV_(R2) and Φ_(VR3) denotes the voltage applied to the gate of thetransistor 11 for resetting the capacitance C_(SH) to referencepotential V_(R3).

V_(C1) denotes the potential of the floating terminal of the storagecapacitor 2.

After bringing the pulse Φ_(VR1) to the high level to turn on the resettransistor 3 for reset operation, X rays as electromagnetic waves areirradiated for a predetermined period of time. Then, after bringing thepulses Φ_(VR2) and Φ_(VR3) up to the ON level and resetting the wiresand the sample hold circuit, the read transistor 4 is turned on byapplying the pulse Φ_(V) to read the signal representing the electriccharge stored in the storage capacitor 2. Subsequently, the aboveoperation will be repeated. X rays may be irradiated continuously.

In FIG. 5, S1 through S5 show changes in the voltage V_(C1) that appearas a result of the changes in the intensity of the electromagneticwaves. When the electromagnetic waves are strong and a large amount ofphoto-generated electric charge is produced, V_(C1) quickly gets to thelevel of saturation voltage V_(SAT) as indicated by S1. On the otherhand, when the intensity of the electromagnetic waves is slightly lowerand the amount of photo-generated electric charge is small, V_(C1)slowly gets to the level of saturation voltage as indicated by S2 andS3. There may be cases where V_(C1) never gets to the level ofsaturation voltage as indicated by S4 and S5 depending on the intensityof electromagnetic waves. Thus, the dynamic range of the device isdefined by voltage V_(R1) and voltage V_(SAT).

As will be understood by comparing the low level voltage of Φ_(VR1) andthat of Φ_(V), +5V is applied to the gate of the reset transistor 3instead of a voltage (e.g., −5V) that completely turns off thetransistor 3 during a charge storing period. In other words, thetransistor 3 is held to an intermediary state between the completely ONstate and the completely OFF state. As a result, the electric charge canflow more easily through the reset transistor 3 than through the readtransistor 4 so that any excessive charge that may be generated can bedischarged through the transistor 3.

The above embodiment is advantageous when the reset transistor 3 and theread transistor 4 are made to substantially have a same configurationand show same threshold values (provided that any minor deviation ofthreshold value less than 1V due to the manufacturing process variationis disregarded). Particularly, the above embodiment operates effectivelywhen all the transistors are formed on a same substrate through a samefilm forming process.

However, the above embodiment may be so modified as to make the resettransistor 3 and the read transistor 4 show different threshold values.For instance, either the channel of the reset transistor 3 or that ofthe read transistor 4 may be doped with an impurity to differentiate theextent of channel doping between the transistor 3 and the transistor 4.Assume that this technique is used and the threshold value of the gatevoltage that completely turns off the reset transistor 3 is made lowerthan −5V while the threshold value of the gate voltage that completelyturns off the read transistor 4 is made equal to −5V. Then, when a samegate voltage (e.g., −5V) is applied to the gate of the reset transistor3 and that of the read transistor 4, the read transistor 4 is completelyturned off whereas the reset transistor 3 is not completely turned offso that it is possible to discharge any excessive electric chargethrough the reset transistor 3 on a priority basis.

FIG. 6 is a timing chart illustrating the operation of the aboveembodiment of electromagnetic wave detector. It is assumed here that theembodiment is continuously irradiated with X rays.

Referring to FIG. 6, D1, D2, . . . , DN indicate the drive operations ofthe respective rows. For instance, D1 indicates the timings of the 1strow. With regard to D1, Φ_(VR11) indicates the reset pulse outputtedfrom the reset circuit 14 and Φ_(V1) indicates the drive pulse fordriving all the lines of the 1st row as outputted from the scan circuit13, while Φ_(H) (Φ_(H1), Φ_(H2), . . . ) indicates the read pulsesoutputted to the output circuit 16 from a scan circuit (not shown). Withthis timing arrangement of operation, the signal is sent out from theoutput terminal OUT to an analog/digital converter circuit (not shown)by way of an external output amplifier and stored in a memory (notshown).

As for D1, the electric potential of each storage capacitor 2 of thefirst row line is reset by pulse Φ_(RESET1) in period T2′ and anoperation of storing an electric charge is started in period T1 so thatthe irradiated X rays are received by the conversion element 1 that maybe an X ray sensor cell and the generated electric charge is stored inthe storage capacitor 2 generally throughout the period T1-T2. In thisembodiment, since the potential of the storage capacitor 2 is lowered bythe generated electric charge, it may be so regarded that the electriccharge stored in the storage capacitor 2 is discharged by the resetoperation. The transistor 4 is turned on by pulse Φ_(DRIVE1) in periodT2 and the electric charge is transferred to the capacitance C₂ of eachrow. Then, the storage capacitor 2 of the first row line is reset bypulse Φ_(RESET1) in the period T2 and a storage cycle is started in thenext period T1″.

In parallel with the storage cycle of the period T1″, the signals of thesignal charges stored in the period T1 are sequentially outputted bypulse Φ_(READ1) from each row to an A/D converter circuit (not shown) byway of an output amplifier. Also in parallel with the storage cycle ofthe period T1″, the signal charges that started to be stored by D2 inthe period T1 are transferred to the capacitances C₂ of the columns.

Thus, the signals of all the rows are read out by D1 through DN. Notethat, in FIG. 6, the all the periods T1′, T1 and T1″ have a same lengthwhile the periods T2′ and T2 have a same length and the periods T3′ andT3 have a same length.

The time length of T1 is 33 msec when images are picked up for 30 framesper second (T1≅{fraction (1/30)} sec). When 500×500 conversion elementsare arranged to a matrix, the reading operation of D1 through D500 isrequired with T2≅T1/500, or about 66 μsec, and T3=T2/500, or about 130nsec.

A read cycle (Φ_(DRIVE1)) of the transistor 4 and a reset (Φ_(RESET1))take place in period T2. The signal has to be fully read because thesensor output level can be lowered otherwise. Time constantt_(read)=C₁·Ron_(R) is defined by the capacitance C₁ of the storagecapacitor 2 and the ON resistance Ron_(R) of the read transistor 4. Avalue of 3t_(read) or more is desired for the signal to be fully read.It is also desired that Φ_(RESET) has a similar time length. Thus, thetime constant of the reset circuit is expressed byt_(reset)=C₁Ron_(Reset). Therefore, it is desired that T2≧3C₁(Ron_(R)+Ron_(Reset)) holds true. If T2=66 μsec, C₁(Ron_(R)+Ron_(Reset))≦22 μsec will be obtained. If, on the other hand, avalue of 5t_(read) or more is used, C₁ (Ron_(R)+Ron_(Reset))≦13 μsecwill hold true. It is also desired that both Ron_(R) and Ron_(Reset) aresufficiently small.

FIG. 7 is a graph showing the characteristic of the ON resistance of athin film transistor (TFT) realized by using amorphous silicon that canbe used for the purpose of the invention. The thin film transistor madeof amorphous silicon may have a configuration as shown in FIG. 3A orFIG. 3B. In FIG. 7, the horizontal axis represents the ratio of thechannel width W to the channel length L(W/L) of the TFT and the verticalaxis represents the ON resistance.

In FIG. 7, the broken line shows the calculated values for a specimenhaving a 300 nm thick semiconductor layer made of non-doped hydrogenatedamorphous silicon (i layer) and the solid line shows the calculatedvalues for a specimen having a 100 nm thick semiconductor layer of thesame material, whereas ▴, ● and Δ indicate the actually measured values.

The capacitance C₁ is typically about several pF. The various timeconstants can be made smaller than 10 μsec with ease by appropriatelydesigning the thin film transistors.

However, the ON resistance of a thin film transistor depends on thepotential difference V_(d) between the source and the drain.

FIG. 8 shows the V_(d) dependency of a specimen having a 500 nm thick ilayer and that of a specimen having a 300 nm thick i layer. The ONresistance rapidly increases as the value of V_(d) falls below 1V.Therefore, the time constants can be improved remarkably by selecting avalue greater than 1V for the potential difference V_(d) between thesource and the drain.

If C₁<<C₂, the capacitor 2 shows an electric potential between V_(R1)/C₂and (V_(B)−V_(th))/C₂. Therefore, the voltage V_(d) between the sourceand the drain of the transistor 4 operating as switch is made greaterthan 1V for both V_(R1)/C₂ and (V_(B)−V_(th))/C₂.

When V_(R2) is so selected as to make |V_(B)−V_(th)|+1|V_(R2)| holdtrue, signals can be read out at high speed by way of the transistor 4under any operating conditions.

FIG. 9 is a schematic circuit diagram of a pixel of another embodimentof electromagnetic wave detector according to the invention, showing itscircuit configuration. This embodiment differs from the above describedembodiment in that transistor 9 is arranged between the conversionelement 1 and the capacitor 2. Otherwise, this embodiment is identicalwith the above embodiment. This embodiment operates in a manner asbriefly described below.

Upon receiving electromagnetic waves in a state where the transistor 9is off, an electric charge is stored in the conversion element 1. Then,the transistor 3 is turned on to reset the capacitor 2. The transistor 4may be turned on immediately thereafter to read out the so-called resetnoise to the output line 5. Alternatively, the reset noise may be readout immediately after reading out the optical signal charge, which willbe described hereinafter. After the elapse of a predetermined period oftime after the start of storing the photo-generated charge in theconversion element in a state where the transistor 9 is off, ON pulseΦ_(T) is applied to the gate of the transistor 9 to turn on the latterand transfer the electric charge to the capacitor 2. At this time, theexcessive charge exceeding the saturation charge of capacitor 2 which ispredetermined in the stage of designing the circuit (and may notnecessarily be the absolute saturation charge that can be stored incapacitor 2) is discharged by way of the transistor 3 to the side of thereference power source that is used to supply a reset potential.

After turning off the transistor 9, the transistor 4 is turned on andthe optical signal charge is read out to the output line 5. The noise ofthe signal can be reduced by subtracting the reset noise determinedabove from the read out signal.

FIG. 10 is a schematic cross sectional view of a typical example ofelectromagnetic wave detector according to the invention.

Referring to FIG. 10, the detector 30 comprises an insulating substrate32 carrying thereon an array of thin film transistors 33. Note that atleast the above described transistors 3 and 4 are like the thin filmtransistors illustrated in FIG. 10.

In FIG. 10, reference numeral 31 denotes a conversion element thatcomprises a semiconductor substrate 40 for receiving electromagneticwaves, a common electrode 41, individual electrodes 39 and insulatingfilms 42. The conversion element 31 is adapted to generate electron-holepairs by the action of the X rays it receives and store either of thecarriers.

Each thin film transistor 33 comprises a gate electrode 43, a channel44, source and drain regions 45 and source and drain electrodes 46. InFIG. 10, reference numeral 34 denotes an interlayer insulating layer andreference numeral 35 denotes a connecting electrode.

The conversion element 31 and the detector 30 are electrically andmechanically connected and secured to each other by means of metallayers 36 and 38 operating as connection pad and bumps 37, although someother form of connection may be used for the purpose of the invention.

The semiconductor substrate 40 is typically formed by using asemi-insulating GaAs monocrystalline substrate and X rays or some otherradiations enter the conversion element by way of the common electrode41 that is typically made of an AuGeNi alloy and held in ohmic contactwith the substrate 40. In this embodiment, radiations are transformedinto an electric charge not by means of a pn junction but in thesemi-insulating substrate. The electrodes 39 typically made of an AuGeNialloy for ohmic contact are electrically connected to the capacitor 2,the reset transistor 3 that is a thin film transistor 33 and the readtransistor 4 that is also a thin film transistor and hence the generatedelectric charge is stored in the capacitor 2.

FIG. 11 is a schematic cross sectional view of another typical exampleof electromagnetic wave detector according to the invention. Thisexample differs form that of FIG. 10 in that the conversion element is aPIN junction diode. More specifically, the diode is formed by using ann+ layer 47, an i layer 40 and a p+ layer 48 that contain GaAs, GaP, Ge,Si or CdTe as principal ingredient. A depletion layer is extended in theentire i layer 40 to facilitate the collection of electric charge.

FIGS. 12A and 12B are a schematic plan view and a schematic crosssectional view of an X ray detector according to the invention.

Referring to FIGS. 12A and 12B, a plurality of conversion elements 31are arranged in the form of a matrix on a common detector 30 comprisingthin film reset transistors and thin film read transistors formed on aninsulating substrate typically made of glass. Each of the conversionelements 31 and the detector 30 are connected to each other by way ofbumps 37.

The signal processing circuit of the device comprises a plurality ofsignal processing circuit chips 50 provided in the form of tape carrierpackages adapted to process signals from a predetermined number ofoutput lines 5 and a common printed wired board 52 for connecting them.Each signal processing circuit chip 50 includes an amplifier 15, anoutput circuit 16 and a transistor 8, which are described earlier.

Similarly, the driver circuit of the device comprises a plurality ofdriver circuit chips 51 provided in the form of tape carrier packagesadapted to drive a predetermined number of drive control lines 6 and 7and a common printed wired board 53 for connecting them. Each drivercircuit chip 51 includes a scan circuit 13 and a reset circuit 14.

The chips 50 and 51 are those of monolithic integrated circuits wheretransistors are formed in a monocrystalline semiconductor substrate.

If polycrystalline thin film transistors or monocrystalline thin filmtransistors are used for the thin film transistors, the signalprocessing circuit and the driver circuit may entirely or partly beformed by using CMOS type thin film integrated circuits comprisingpolycrystalline thin film transistors or monocrystalline thin filmtransistors arranged on the substrate 32 in such a way that they areintegrated with a plurality of unit cells on the substrate 32. Thisarrangement is advantages that it can reduce the number of connectionterminals to be used externally relative to the substrate 32 toconsequently simplify the assembling operation.

In FIGS. 12A and 12B, reference numeral 54 denotes a single sheet ofconductor for short-circuiting the plurality of conversion elements 31and commonly biassing them. While the conductor 54 is in the form of asheet here, it may alternatively realized in a meshed form. Referencenumeral 55 denotes an insulating sheet and reference numeral 56 denote asheet for shielding the biassing conductor. A high voltage above 100V isapplied to the conductor 54 and hence requires a protection sheet 56.Particularly, when the detector is used for medical applications, theprovision of the sheet 56 is highly desirable so that the conductor towhich a high voltage is applied is held remote from any human body.

The insulating sheet 55 may not necessarily be arranged between theconductor 54 and the sheet 56. It may be replaced by an air gap. If suchis the case, the shield 54 is arranged between the conductor and thehousing of the detector.

FIGS. 14A and 14B of the accompanying drawings show for the purpose ofcomparison a schematic cross sectional view and a schematic plan view ofa detector having a layered structure of monocrystalline bulk X raydetecting sections and monocrystalline read Ics. Since the detectorcomprises an upper substrate and a lower substrate, which aremonocrystalline substrates, it involves a complex wiring arrangement andis not adapted to up-sizing. On the other hand, any of the abovedescribed embodiments of the present invention are adapted to up-sizing.

The detector of FIGS. 14A and 14B is disadvantageous in that both theupper and lower substrates are accompanied by a multilayered wiringarrangement and it involves a complex manufacturing process with a largenumber of manufacturing steps to consequently reduce the manufacturingyield. Then, the wires can show a large floating capacitance to lowerthe detection speed of the device and reduce the electric gain. To thecontrary, any of the above described embodiments of the presentinvention are free from these drawbacks because they utilizes thin filmtransistors formed on an insulating substrate.

Thus, the above described embodiments of electromagnetic wave detectoraccording to the invention provides the following advantages.

(1) Any after image can be eliminated by the use of thin film resettransistors.

(2) The saturation voltage V_(S) of the storage capacitor 2 can beselected to be equal to the difference of the OFF voltage of the thinfilm reset transistor and the threshold voltage, or V_(B)−V_(th) so thatany leakage of electric charge to the output line that can otherwisearise in response to an excessive input can be effectively preventedfrom occurring.

(3) Since the substrates of the conversion elements of the device arearranged two-dimensionally and a large common insulating substrate islaid thereon for the detector, an imaging device adapted to cover alarge area can easily be prepared.

(4) Even if amorphous semiconductor thin film transistors showing a lowcarrier mobility are used, a moving image can be picked up effectively.Additionally, a still image can be picked up with an enhanced level ofsensitivity and a wide dynamic range.

(5) The response time of the detector can be reduced by appropriatelyselecting a reset potential to make the potential difference between thesource and the drain of the thin film read transistor at least greaterthan 1V.

(6) Since a detector according to the invention provides a highsensitivity and a wide dynamic range, it can find applications in thefield of analyzers for analyzing both living things and non-livingthings and that of non-destructive testers in addition to medicalapplications.

(7) Since a detector according to the invention uses thin filmtransistors, the risk of operation errors that can be causedunintentionally by high energy rays can be significantly reduced.

FIG. 13 is a schematic block diagram of a medical diagnostic systemcomprising an imaging device realized by using an electromagnetic wavedetector according to the invention.

Referring to FIG. 13, there are shown an X ray tube 1001 for generatingX rays, an X ray shutter 1002 for controlling the X ray path by closingand opening it, an irradiation sleeve or a movable stop 1003, a subjectof examination 1004, a radiation detector 1005 realized by using anelectromagnetic wave detector according to the invention, a dataprocessing unit 1006 for processing the signals from the radiationdetector 1005 as data and a computer 1007, which is adapted to displaythe X ray image obtained on the basis of the signals from the dataprocessing unit 1006 on a display 1009 such as a CRT and control therate of generation of X rays by controlling the X ray tube 1001 by wayof camera controller 1010, X ray controller 1011 and capacitor type highvoltage generator 1012.

High energy rays such as X rays vary enormously in terms of the amountof energy entering the conversion elements between those that havepassed through the subject and those that have been transmitted throughair without passing through the subject. Therefore, the electric chargegenerated by X rays will also vary enormously between them. Therefore,the stored electric charge can easily be saturated in the backgroundarea due to the difference in the generated electric charge between thesubject and the background. However, according to the invention, sinceany excessive charge can be discharged through thin film transistors,the degradation of the image quality due to such an excessive charge canbe effectively avoided. Additionally, since thin film transistors areused, the risk of operation errors that can be caused unintentionally byhigh energy rays entering the thin film transistors can be significantlyreduced. Still additionally, a detector that can cover a large area caneasily be realized.

Since an electromagnetic wave detector according to the invention caneffectively prevent any after image and leakage of electric charges fromtaking place and operate without errors when high energy electromagneticwaves enter it, it can effectively detect electromagnetic waves even ifthey show a high energy level and hence operates excellently if comparedwith conventional devices. Finally, it is possible to provide anelectromagnetic wave detector having a large imaging area at low costthan ever.

1. An electromagnetic wave detector comprising: conversion elements forconverting incident electromagnetic waves or radiations into an electriccharge; storage capacitors for storing the electric charge produced bysaid conversion elements; thin film read transistors connectedrespectively to the corresponding storage capacitors, each of said readtransistors having a threshold voltage and a gate to which first andsecond voltages are applied respectively in readout and storage periods,wherein said first voltage is higher than said threshold voltage of saidread transistor; and thin film reset transistors connected respectivelyto the corresponding storage capacitors, each of said reset transistorshaving a threshold voltage and a gate to which third and fourth voltagesare applied respectively in reset and storage periods wherein said thirdvoltage is higher than said threshold voltage of said reset transistor,and wherein a gate potential of said read transistors and a gatepotential of said reset transistors can be independently controlled, andthe difference between the fourth voltage and the threshold voltage ofsaid reset transistors is smaller than the difference between the secondvoltage and the threshold voltage of said read transistors.
 2. Anelectromagnetic wave detector according to claim 1, wherein saidconversion elements are adapted to absorb electromagnetic waves showingan energy level higher than visible light and convert them into anelectric charge.
 3. An electromagnetic wave detector according to claim1, wherein said thin film read transistors and said thin film resettransistors have a non-monocrystalline semiconductor layer formed on aninsulating substrate.
 4. An electromagnetic wave detector according toclaim 1, wherein said thin film read transistors and said thin filmreset transistors are formed on an insulating substrate, and whereinsaid conversion elements are formed on a substrate different from saidinsulating substrate and electrically connected to said thin film readtransistors and said thin film reset transistors.
 5. An electromagneticwave detector according to claim 1, wherein said conversion elementscomprise a semiconductor substrate having two opposite surfaces forconverting electromagnetic waves into an electric charge, a commonelectrode arranged on the one surface of the semiconductor substrate anda plurality of electrodes formed on the other surface of thesemiconductor substrate and separated from each other in correspondenceto a plurality of two-dimensional pixels, wherein said thin film readtransistors and said thin film reset transistors are formed on aninsulating substrate such that unit cells each including one of the thinfilm read transistors and one of the thin film reset transistors arearranged on the insulating substrate in correspondence to the pixels,and wherein said semiconductor substrate and said insulating substrateform a layered structure and said plurality of electrodes and said unitcells are electrically connected between the substrates.
 6. Anelectromagnetic wave detector according to claim 5, wherein saidsemiconductor substrate is provided in plurality as arrangedtwo-dimensionally on said insulating substrate to form a layeredstructure and the common electrodes of the semiconductor substrates aremutually short-circuited.
 7. An electromagnetic wave detector accordingto claim 5, wherein a high voltage is applied to the common electrode ofsaid conversion elements and a shielding conductor is arranged near thecommons electrode.
 8. An electromagnetic wave detector according toclaim 1, wherein said thin film read transistors and said thin filmreset transistors are formed on an insulating substrate provided with adriver circuit for driving the thin film read transistors and the thinfilm reset transistors and with a read circuit for reading signals fromsaid thin film read transistors.
 9. An electromagnetic wave detectorcomprising: conversion elements for converting incident electromagneticwaves or radiations into an electric charge; storage capacitors forstoring the electric charge produced by said conversion elements; andthin film reset transistors connected respectively to the correspondingstorage capacitors and each having a gate to which first and secondvoltages are applied respectively in reset and storage periods, whereinany excessive electric charge is discharged by way of said thin filmreset transistors in each storage period.
 10. An electromagnetic wavedetector according to claim 9, wherein said conversion elements areadapted to absorb electromagnetic waves showing an energy level higherthan visible light and convert them into an electric charge.
 11. Anelectromagnetic wave detector according to claim 9, wherein said thinfilm reset transistors have a non-monocrystalline semiconductor layerformed on an insulating substrate.
 12. An electromagnetic wave detectoraccording to claim 9, wherein said thin film read transistors and saidthin film reset transistors are formed on an insulating substrate, andwherein said conversion elements are formed on a substrate differentfrom said insulating substrate and electrically connected to said thinfilm read transistors and said thin film reset transistors.
 13. Anelectromagnetic wave detector according to claim 9, wherein saidconversion elements comprises a semiconductor substrate having twoopposite surfaces for converting electromagnetic waves into an electriccharge, a common electrode arranged on the one surface of thesemiconductor substrate and a plurality of electrodes formed on theother surface of the semiconductor substrate and separated from eachother in correspondence to a plurality of two-dimensional pixels,wherein thin film read transistors and said thin film reset transistorsare formed on an insulating substrate such that unit cells eachincluding one of the thin film read transistors and one of the thin filmreset transistors are arranged on the insulating substrate incorrespondence to the pixels, and wherein said semiconductor substrateand said insulating substrate form a layered structure and saidplurality of electrodes and said unit cells are electrically connectedbetween the substrates.
 14. An electromagnetic wave detector accordingto claim 13, wherein said semiconductor substrate is provided inplurality as arranged two-dimensionally on said insulating substrate toform a layered structure and the common electrodes of the semiconductorsubstrates are mutually short-circuited.
 15. An electromagnetic wavedetector according to claim 13, wherein a high voltage is applied to thecommon electrode of said conversion elements and a shielding conductoris arranged near the common electrode.
 16. An electromagnetic wavedetector according to claim 9, wherein thin film read transistors andsaid thin film reset transistors are formed on an insulating substrateprovided with a driver circuit for driving the thin film readtransistors and the thin film reset transistors and with a read circuitfor reading signals from said thin film read transistors.